1. Field of the Invention
The present invention relates to a planetary gear and a method for manufacturing the same, and in particular to a planetary gear including a pinion shaft for supporting a pinion rotatably in which an internal lubricating oil path is perforated into the pinion shaft from an outer peripheral surface obliquely with respect to the axis line.
2. Description of the Related Art
A prior art technique of this kind is disclosed, for example, in Japanese Patent Laid-Open Publication No. 58-8849. Specifically, in this prior art technique, a planetary gear includes a pinion shaft for rotatably supporting a pinion, a planet carrier for holding the pinion shaft and provided rotatably so that the pinion can be revolved, a thrust washer interposed between a carrier plate for supporting the pinion of the planet carrier and an axial end surface of the pinion. Furthermore, an annular groove is formed in an inner peripheral surface of the carrier plate, and a path is formed in a joining surface portion between the carrier plate and the thrust washer so that the path passes through in the vicinity of the pinion shaft and the annular groove, and a through hole is formed within the pinion shaft to extend in an oblique direction so that the the path is communicated with the outside of an outer peripheral surface of the pinion shaft. In such a structure, a lubricating oil path can be formed without increasing the number of parts.
In the case of forming the internal lubricating oil path extending in an oblique direction with respect to the axis line of the pinion shaft, by forming an aperture in the outer peripheral surface of the pinion shaft in a similar manner as the through hole within the pinion shaft disclosed in the above-mentioned patent publication, in order to make easy the formation of an internal lubricating oil path having a small diameter, it is general to form a lower hole in the outer peripheral surface of the pinion shaft at a position intended to be an aperture of the internal lubricating oil path so that the lower hole is substantially in parallel with the internal lubricating oil path. Thus, for example, as shown in FIG. 7, a lower hole 52 is perforated into the outer peripheral surface of the pinion shaft 50 at a position near the axial end portion by means of an end mill 51 from an oblique direction so that a bottom portion 52a of the lower hole 52 is located nearer to the axial center portion than the aperture portion. Following this, it is considered to perforate an internal lubricating oil path 53 having a small diameter in an oblique direction by utilizing the bottom portion 52a of the lower hole 52.
However, in this structure in which the lower hole 52 is formed substantially parallel with the internal lubricating oil path 53, when an end mill 51 having a large diameter is used, as shown in FIG. 8, which is a view as viewed in the direction of the arrow A in FIG. 7, a portion of the lower hole 52 having a maximum diameter Rmax will be positioned near the axial center portion of the pinion shaft 50. On the other hand, the outer peripheral surface of the pinion shaft 50 may be divided into the following three areas from the axial end surface side as shown in FIG. 8: a holding area 50A held by the carrier plate, a washer sliding area 50B around which the thrust washer is fitted slidably, and a bearing area 50C around which a bearing is fitted to make the pinion rotatable. Furthermore, it is required that lubricating oil be fed into the the lower hole 52. Since it is easy to form an oil path in the carrier plate, the lower hole 52 is apt to be formed near the holding area 50A.
Then, the portion of the lower hole 52 having the maximum diameter Rmax will be located, for example, within the washer sliding area 50B. However, if so, a convex portion of convex and a concave portion, formed in an inner peripheral surface of the thrust washer to enable the lubricating oil to move easily to the outside and inside of the bearing, will tall into the lower hole 52 during sliding movement of the thrust washer. As a result, there is a drawback that the convex portion of the thrust washer and a corner portion of the lower hole 52 will be broken. Moreover, if the convex portion and the corner portion of the lower hole 52 are broken, their broken fragments might fall into gaps of the thrust washer and the bearing, and there is a possibility that the smooth rotation of the pinion will be disturbed. Furthermore, it is also considered to perforate the lower hole 52 by using an end mill having a large diameter. However, as shown in FIG. 9, which is a sectional view taken along the line 9--9 in FIG. 7, the aperture portion of the lower hole 52 (in particular, a portion of a maximum diameter Rmax) will be enlarged, and the lubricating oil entering into the lower hole 52 will easily disperse due to an inertia force to the left and right (or to opposite directions) as shown by the arrows in FIG. 9. It also will be difficult to get the lubricating oil to flow into the internal lubricating oil path 53, resulting in degradation of the lubrication efficiency. In addition, in the case where the end mill 51 has a large diameter, the amount of cutting of the pinion shaft will be increased relatively, and the strength of the pinion shaft 50 will be degraded. Therefore, the diameter of the pinion shaft must be increased.
Furthermore, depending on the position at which the lower hole 52 is formed, it is also considered that the portion having the maximum diameter Rmax is located within the bearing receiving area 50C serving as a rolling surface of a rolling member, and in such a case, the strength of the rolling surface will be varied to a great extent depending on a position in a circumferential direction. For example, a force in a bending direction will be imparted to the rolling member periodically, and the durability of the rolling member will be lowered.
In order to remove such drawbacks, it is also considered to perforate the lower hole 52 by using an end mill having a small diameter. However, when the lower hole 52 is perforated by using the end mill having a small diameter, as shown in FIG. 10, which is a similar sectional view to FIG. 9, thin wall portions 52b will be formed to the left and right sides of the aperture portion of the lower hole 52, and since the thrust washer slidably moves or the rolling member of the bearing rolls on the thin wall portions 52b, also the pinion shaft 50 will be easily broken.